1. Field of the Invention
The present invention relates in general to a holographic interference exposing device, and more particularly to a holographic interference exposing device suitable for used in preparation of a diffraction grating of a fixed period of several thousand Angstroms used for varieties of optical systems, such as spectrometers, interference systems, semiconductor lasers and optical communication systems.
2. Description of the Prior Art
Conventionally, a holographic interference exposing device has been used for patterning a semiconductor device by forming an interference fringe using a difference of light paths of laser beams.
With reference to FIG. 1, there is shown a holographic interference exposing device (hereinafter, referred to simply as "the known exposing device") in accordance with an embodiment of the prior art. In this drawing, the reference numeral 1 denotes a laser source, each of the numerals 2, 3, 5 and 6 denotes a mirror, the numerals 4 denotes a half mirror or a beam splitter and the numeral 7 denotes a semiconductor substrate.
As shown in this drawing, the beam splitter 4 of the known exposing device splits the laser beam of a short wavelength generated by the laser source 1 into two laser beams which are to pass along different light paths, respectively. Thereafter, the known exposing device generates a light and shade fringe having a predetermined period using the difference of the light paths of the two laser beams.
In the process for preparation of the desired semiconductor device using the fringe, the grating of a micro period of about 2,000.ANG. is generated. Here, the exposing laser source 1 generates ultraviolet laser beam which has a short wavelength as well as a sufficient amount of energy for developing the photoresists on the semiconductor substrate 7.
The laser beam generated by the laser source 1 is orderly reflected by the mirrors 2 and 3 prior to reaching the beam splitter 4 where the laser beam is split into two laser beams. The two laser beams split by the beam splitter 4 are in turn reflected by individual mirrors 5 and 6 to be focused on the surface of the semiconductor substrate 7. When the two laser beams are focused on the surface of the substrate 7, the interference fringe is formed on the surface of the substrate 7.
This fringe is used in developing the photoresists on the semiconductor substrate 7 to form a supermicro pattern on the semiconductor substrate 7. That is, the photoresists 8 having the grating of a given period 12 are developed on the substrate 7 as shown in FIG. 2.
Turning to FIG. 3, there is shown in a sectional view the substrate 7 which has been subjected to an etching treatment using the photoresists 8 developed on the substrate 7.
However, the aforementioned exposing device has several problems. That is, an invisible laser source, such as an ultraviolet laser source or an infrared laser source, has been used as a laser source of varieties of interference systems, including a holographic interference exposing system, and of spectrometers used in preparation of a supermicro diffraction grating. Here, the problem is caused by the fact that the laser beam is invisible and this makes the alignment of the optical system quite difficult. In order to overcome such a problem caused by the invisible laser beam, a visible laser source, such as a He-Ne gas laser source generating a red laser beam or a semiconductor laser source, may be installed in the housing of the main laser source such that it positioned at the same place as the main laser source. In addition, the visible laser source may be positioned at a place spaced apart from the main laser source while making the light path of the visible laser beam generated thereafter combine or superimpose with that of the invisible laser beam generated by the main laser source. However, to place the invisible laser source and the visible laser source at the same position requires a special technique and this causes the laser source to be very expensive. On the other hand, to place the visible laser source at a position spaced apart from the invisible laser source causes a problem in manufacture of the optical system due to the vibration difference between the positions of the respective laser sources.